F# / .NET: Gotcha's
This is another annoying "feature" in .NET. For some reasons, TimeSpan.ToString with a custom format string does all sorts of wonky stuff, particularly it will give you the "Invalid Format String" error on any unescaped character that isn't a valid format specifier.
Basically, take this list: Custom TimeSpan Format Strings and treat it literally.
Want to add a space between days and hours/minutes/seconds? Escape it: d\ hh\:mm\:ss. Yes, even spaces.
F# / .NET: Gotcha's
This is one of the biggest, most painful errors I've ever seen in my life. There's absolutely no direction this error can take you.
Here's the thing: GDI+ doesn't really report any errors it encounters down the stack properly. They're all "Generic Errors".
So, some things to look for:
- Is the resource disposed? If you have a
Using / using / use, and you try to access the resource outside of that scope, you'll probably get this error.
- Calling to
.Save? There's a whole list of stuff for that. If you get this error when calling .Save on an Image / Bitmap / etc., you might want to check any and all of the following:
- Does the directory exist? GDI+ won't create it.
- Do you have permission to the directory / file? GDI+ won't tell you if that's the problem.
- Did you dispose of the
MemoryStream? That'll be a generic error. You'll want to make sure you save before disposal.
There are many, many more, and I'll be updating this list as time permits.
F# / .NET: Gotcha's
If you are a user of Giraffe with ASP.NET Core and F#, you might have come across the following error:
FS0708: This control construct may only be used if the computation expression builder defines a 'Bind' method
This happens if you try to use a let! expression inside a task computation expression:
task {
let! x = ...Async()
The problem here is that task as a computation expression doesn't define a "Bind" method, from what I can tell. There is, however, a quick workaround:
open FSharp.Control.Tasks.V2.ContextInsensitive
For some reason, this open fixes it. I assume that it allows the async computation-expression version of the Bind to be used, so that the code above works.
There's been discussion around this, from what I can tell, but it hasn't really resolved this issue, not sure if it's a version thing, or entirely unrelated: Q: remove async await bind from task {}?.
The Giraffe folks do a great job, so I want to make sure you can work around this particular issue if and when you encounter it.
Recently I've been digging more-and-more into F#, so I want to start putting together a list of things that I occasionally (or regularly) run into that new users of the language (or even those who are substantially experienced) might not have a great time with.
I'll be notating if it's a general .NET "gotcha", or an F#-specific "gotcha."
F#:
.NET:
If you have something you want to see in the list, please let me know either via Twitter or as a comment. I'll try to check back here regularly to keep this list as up-to-date as possible.